Thermal degradation kinetics of poly(propylene succinate) prepared with the use of natural origin monomers

Thermal degradation kinetics of poly(propylene succinate)

prepared with the use of natural origin monomers*

Paulina Parcheta1)_{, Iwona Koltsov}2)_{, Ewa Głowińska}1)_{, Janusz Datta}1), _**) DOI: dx.doi.org/10.14314/polimery.2018.10.6

Abstract: Linear bio-based polyester polyols were prepared with the use of succinic acid and 1,3-pro-panediol (both with natural origin). Tetraisopropyl orthotitanate (TPT) was used as a catalyst. In order to determine the effect of various synthesis temperature conditions on the thermal degradation kine­ tics, nine sequences of temperature conditions were used during two-step polycondensation reaction. Thermogravimetric analysis was conducted with the use of DSC­TG/QMS method (differential scanning calorimetry­coupled with thermogravimetry and quadrupole mass spectrometry). The results indicated high thermal stability of the obtained materials. They undergo a one-step thermal decomposition with the temperature of maximum rate of weight loss at ca. 405 °C. Moreover, the thermal degradation kine­ tics was determined with the use of Ozawa, Flynn and Wall as well as Kissinger methods. The highest thermal degradation activation energy was equal to 196.4 kJ/mol.

Keywords: poly(propylene succinate), bio­based polyester, thermal degradation kinetics, Ozawa, Flynn and Wall method, Kissinger method.

Kinetyka degradacji termicznej poli(bursztynianu propylenu)

zsyntetyzowanego z monomerów pochodzenia naturalnego

Streszczenie: Liniowe bio­poliole poliestrowe syntetyzowano z wykorzystaniem substratów pochodze-nia naturalnego: kwasu bursztynowego oraz 1,3­propanodiolu. W charakterze katalizatora stosowano ortotytanian tetraizopropylu (TPT). W celu określenia wpływu temperatury syntezy na kinetykę de-gradacji termicznej, podczas dwuetapowej reakcji polikondensacji zastosowano różne warunki tempe-raturowe w dziewięciu sekwencjach. Analizę termograwimetryczną prowadzono za pomocą metody różnicowej kalorymetrii skaningowej sprzężonej z termograwimetrią i kwadrupolową spektrometrią masową (DSC­TG/QMS). Wyniki badań potwierdziły dużą stabilność termiczną materiałów oraz jedno-etapowość procesu rozkładu temperaturowego z temperaturą maksymalnego rozkładu wynoszącą ok. 405 °C. Określono też kinetykę degradacji termicznej metodami Ozawy, Flynna i Walla oraz Kissingera. Największa wartość energii aktywacji degradacji termicznej wyniosła 196,4 kJ/mol.

Słowa kluczowe: poli(bursztynian propylenu), bio­poliole poliestrowe, kinetyka degradacji termicznej, metoda Ozawy, Flynna i Walla, metoda Kissingera.

The primary reaction, which leads to the obtainment of polyester polyols, is a well­known two­step polycon-densation reaction. The first step constitutes the esterifi-cation or transesterifiesterifi-cation reaction between carbo xylic

acid or carboxylic acid esters and the excess of the gly-cols. During the reaction, such by-products as water or alcohols, respectively, are formed. For shifting the reac-tion towards the main product, the by-product must be removed from the reaction mixture. The capability of the by­product elimination affects the reaction kinetics and productivity. After the by­product is removed, the second step – polycondensation reaction, can be started [1]. It is well­known that the reaction kinetics are also affected by the amount and chemical structure of the catalyst and monomers and by the temperature during both steps and reaction time [2]. The polyurethane materials obtained with the use of polyester polyols are less resistant to hy-drolysis compared to the polyether polyols. However, it makes them more favorable due to the biodegradabili-ty [3–5]. Polyurethanes (PUR) based on polyester

poly-1) _{Gdańsk University of Technology, Faculty of Chemistry,} Department of Polymers Technology, G. Narutowicza 11/12, 80­233 Gdańsk, Poland.

2) _{Polish Academy of Sciences, Institute of High Pressure} Phy-sics, Laboratory of Nanostructures for Photonic and Nanome-dicine, Sokołowska 29/37, 01­142 Warszawa, Poland.

*) _{Material contained in this article was presented at the} Scien-ce and Technology ConferenScien-ce ”Polyurethanes 2017 – mate-rials friendly to humans and environment”, Ustroń, Poland, 8–11 October 2017.

ols have better thermal and fire resistance as well as su-perior solvent resistance than the polyether­based PUR. Furthermore, polyesters provide major possibilities for the preparation of biorenewable PUR material [6].

Currently, the biocomponents which allow produc-ing polyester polyols derived entirely from bioresourc-es are readily accbioresourc-essible [7]. One of the most important bio-based monomers for the synthesis of polyester poly-ols is succinic acid (SA). Since 2012, this compound can be obtained by a biotechnological process through the corn fermentation using such microorganisms as a fungi, yeasts or bacteria [8–11]. After fermentation, the product is purified which allows for a purification level of even 99.5 % [12]. The succinic acid based on the biomass fer-mentation is commercially available on a large­scale from such companies as BioAmber [13], Reverdia [14], Myriant [15] and BASF/Purac [16].

The second monomer taking part in the polyconden-sation reaction is a glycol. Bio-based glycol with the highest global usage is 1,3­propanediol (PDO) (Susterra, DuPont) [17]. Biosynthesis pathway for the bio-based PDO preparation involves the single­step fermentation process based on the glucose, sucrose, dextrose, and bio-mass sugars [18, 19]. The most commonly used bacteria to produce 1,3-propanediol are Klebsiella [18], Clostridia [20],

Citrobacter fruendii [21], etc. These microorganisms allow

for the industrial production of PDO with the 99.97 % purity [22, 23]. Other commonly available glycols made from renewable resources are bio-based 1,4-butanediol (bio­BDO) and ethylene glycol (bio­EG). Currently, the research on the bio-based 1,6-hexanediol (bio-HDO) and adipic acid (bio­AA) are carried out [13, 24, 25].

With the use of above­mentioned substances, we are able to synthesize fully bio-based polyester polyols with designed macromolecular structure and properties tai-lored to specific industrial requirements.

Recently, more and more substances can be obtained from biorenewable resources and substitute petrochem-ical­based counterparts. Researchers work on the syn-thesis of the bio-based polyester polyols with the use of these substances. Lu et al. [26] synthesized bio-based polyesters based on the bio-based 1,5-pentanediol and aliphatic diacids with 4, 5, 6, 9, 10, or 12 carbon atoms. Partially bio-based polyester were compared in terms of effects of dicarboxylate chain length on the crystal-line structure and thermomechanical properties. All the polyesters are semicrystalline polymers, where the crystallization degree and melting temperature increase with dicarboxylate chain length. The results indicated also that all polyesters have sufficient thermal stability. Munoz­Guerra et al. [27] investigated bio­based aromatic polyesters prepared by ring-opening polymerization of cyclic ethylene and butylene 2,5-furandicarboxylate oli-goesters. The polymerization of this two compounds led to furan­based polyesters: poly(ethylene furanoate) (PEF) and poly(butylene furanoate) (PBF). It was found that the oligo(butylene 2,5-furandicarboxylate) cycles are more

reactive during polymerization than the ethylene ones, which required higher reaction temperature to reach similar conversions. Papageorgiou et al. [28] investigated synthesis of poly(propylene-2,5-furandicarboxylate) (PPF) as a new bio-based aromatic polyester. They described a comparative study of the thermal behavior and solid state structure of PPF, poly(propylene terephthalate) (PPT) and poly(propylene naphthalate) (PPN). The results indicated that macromolecular chains of PPF and PPT were rigid, due to their glass transition temperatures and thermal sta-bility of these polymers was similar. The melting point of PPF was found at 180 °C when PPN and PPT revealed the melting point at higher temperatures, respectively 207 and 231°C. Zhou et al. [29] studied aromatic polyesters synthesized from 2,5­furandicarboxylic acid (FDCA) and 1,4-butanediol (BDO), which were used for preparation of copolymer with poly(tetramethylene glycol) (PTMG). The results showed that glass transition temperature, melt-ing point, melt crystallization tempe rature and crystal-lization ability decreased with increasing PTMG content. Moreover, PBF­PTMG copolymers exhibited good stress at break and outstanding elongation at break.

A range of articles describing bio­based polyurethane materials were published over the last decade. Petrović et

al. [30] synthesized fast-responding shape-memory

poly-urethanes with the use of bio-based polyester polyols. Polyester polyols were synthesized with the use of 9­hy-droxynonanoic acid and hexanediol. Dicarboxylic acid was obtained by ozonolysis of fatty acids extracted from soy oil and castor oil. The researchers indicated that due to the high crystallization rate of the soft segment, the obtained polyurethanes were characterized by unique properties suitable for shape-memory applications, such as adjustable transition temperatures and good mechani-cal strength. Moreover, they claimed that these materi-als were potentially biodegradable and biocompatible, which make them suitable for biomedical and environ-mental applications. Datta and Głowińska [31] investi-gated bio-based polyurethanes synthesized with the use of vegetable­oil based polyols. In their study, they used a mixture of commercial polyether and hydroxylated soybean oil with different ratios. Furthermore, they used two bio-based low molecular weight glycols: 1,2-propane-diol and 1,3-propane1,2-propane-diol as chain extenders. The results of thermomechanical analysis showed that the polyure-thanes produced with bio-based 1,2-propanediol exhibit-ed higher storage modulus and lower loss modulus than polyurethanes based on 1,3-propanediol as a chain ex-tender. Moreover, they prepared bio­based polyurethane composites with microcrystalline cellulose [32, 33]. They confirmed good interfacial adhesion between the partial-ly bio-based matrix and biofiller. The results of thermo-mechanical analysis showed a positive effect of the filler on the storage and loss modulus of the composites. The tensile strength and elongation at break decreased with increasing filler content, but the addition of microcrys-talline cellulose improved the hardness of the obtained

materials. Saralegi and Eceiza et al. [34] investigated seg-mented thermoplastic polyurethane materials based on the vegetable oil­based polyesters and corn sugar­based chain extender. They studied the effect of soft segment chemical structure and molecular weight on the morpho-logy and properties of the final products. The results in-dicated that chemical structure and molecular weight of polyols strongly affect the properties of the synthesized polyurethanes. With the increasing soft segment molecu-lar weight, the degree of segment crystallinity and mi-crophase separation also increased, which gave enhanced mechanical properties and higher thermal stability.

One of the important features of polyester components is their good thermal stability, which promises to ensure a suitable behavior of the polyols during industrial pro-cessing. By conducting the measurements of thermal sta-bility at various heating rates, the thermal degradation kinetics can be determined.

There are some methods which allow measuring the kinetics of the thermal decomposition. The first method is the Kissinger method [35]. This method makes it pos-sible to determine the activation energy E without the precise knowledge about the mechanism of the reaction in accordance with the Equation (1):

(1) where: β – heating rate [K/min], T_p – temperature corre-sponding to the inflection point (maximum reaction rate) of the thermal degradation curves [K], R – gas constant [8.314 · 10-3 _{kJ/(mol · K)].}

Activation energy E of the decomposition can also be calculated by using isoconventional method of Ozawa, Flynn and Wall (OFW) [36]. This method consists of mea-surements of the temperatures, which are attributable to the constant value of conversion α from experiments car-ried out at different heating rate β. The assumption for this method is that the conversion function f(α) is constant for all values of conversion α with the alteration of the heating rate β. Plotting ln(β) against 1/T according to the Equation (2) allows determining the activation energy E.

(2)

where: β – heating rate [K/min], A – pre-exponential factor, that is assumed to be temperature-independent, f(α) – conversion function, α – conversion value, T – tem-perature [K], R – gas constant [8.314 · 10-3 _{kJ/(mol · K)].}

Ozawa, Flynn and Wall developed also a model­free method for degradation kinetics study with the use of TG data [37]. This isoconversional method uses the fol-lowing Equation (3):

(3) where: g(α) – the integral reaction model, E_app – the ap-proximate activation energy.

At each fixed degree of conversion α, plotting log β against 1/T creates linear trends. The slope of the plot’s best­fit line is proportional to the approximate activation energy, according to the relation (4):

slope = (4)

In the present work, the synthesis of a series of linear bio-based aliphatic polyester polyols is described. The syntheses were designed to obtain the polyesters with proposed weight average molecular weight ca. 2000 g/mol and functionality f = 2. Tetraisopropyl orthotitanate (TPT) was used as a polycondensation catalyst. Thermal degra-dation characteristics of the obtained poly(propylene suc-cinate)s was determined by the use of thermogravimetric analysis (TGA). Kinetics of the thermal degradation was determined by using Ozawa, Flynn and Wall as well as Kissinger methods.

EXPERIMENTAL PART Materials

– Bio­based succinic acid (SA) (solid, molecular weight: 118.09 g/mol, purity: 98–100 %, relative density at 20 °C: 0.900 g/cm3_{) used in this study was obtained from} BioAmber Sarnia Inc. (Ontario, Canada).

– Susterra propanediol (1,3­propanediol) (liquid, mo-lecular weight: 76.09 g/mol, purity: 99.98 %, water con-tent by Karl Fischer: 12.1 ppm, relative density at 20 °C: 1.053 g/cm3_{, dynamic viscosity at 20 °C: 52 mPa · s) was} ob-tained from DuPont Tate&Lyle Corporation Bio Products (Loudon, Tennessee, USA).

– Tetraisopropyl orthotitanate, Ti(O­i-Pr)₄ (TPT), (liq-uid, molecular weight: 284.22 g/mol, purity: 97 %) used as a catalyst was purchased from TCI Chemicals (India). – All other materials and solvents used for analytical measurement methods for characterization of prepared bio-based polyester polyols were of analytical grade. Bio-based polyesters synthesis

Aliphatic bio­based polyester polyols were prepared with the use of dicarboxylic acid, which was succi nic acid, and glycol, 1,3-propanediol. Both used components were of natural origin. The catalyst was used in the same amount, 0.25 wt %, for all of the polyols. The cata-lyst mass was calculated as a glycol equivalent. All lin-ear bio-based polyester polyols were synthesized in the bulk by two­step polycondensation method (esterifica-tion and polycondensa(esterifica-tion). The first step was

represent-ed by the esterification reaction between succinic acid (SA) and 1,3­propanediol (PDO). Glycol was always used in excess. The molar ratio SA : PDO equaled 1 : 1.2, was determined considering the final molar mass expected after full polycondensation (approximately number aver-age molecular weight: M_n = 2000 g/mol and functionality:

f = 2). Both of the steps were carried out in the glass

reac-tor, which consisted of three­neck flask equipped with nitrogen/vacuum inlet, mechanical stirrer, thermometer, and condenser. The glass reactor was placed into a heat-ing mantle. The first step of the reaction was conducted under a nitrogen atmosphere. Succinic acid and 1,3­pro-panediol mixture (without catalyst) was stirred at 140, 150 or 160 °C, depending on the synthesis, and kept at this temperature, until at least 60 % of water as a by­product was received (application for patent in the Polish Patent Office, no. P418808). After the water distillation, the flow of nitrogen was stopped, the appropriate amount of cata-lyst was added to reaction mixture and the temperature was increased up to 160, 180, 190 or 200 °C, depending on the synthesis, under reduced pressure. The value of pres-sure amounted to ca. 6.67 kPa. During polycondensation, the acid number was measured. After achieving the acid number value of ca. or preferably below 1 mg KOH/g, the polycondensation was finished. The values of hydroxyl numbers have to range from 50 to 80 mg KOH/g – the scope of hydroxyl number of polyols dedicated for cast polyurethane elastomers.

Methods of testing

Acid and hydroxyl number

– Carboxyl end­group value measurements were per-formed in accordance with the Polish Standard PN­86/ C45051. Samples about 1 g of the prepared polyesters were dissolved in ca. 30 cm3 _{of acetone at room temperature.} Thereafter, the solutions were titrated with the use of a standard solution of potassium hydroxide KOH in distilled water (0.1 mol/dm3_{) and phenolphthalein as indicator.}

– Hydroxyl group determination was performed with the use of sample about ca. 0.5 g of polyester. The sample was dissolved in 5 cm3 _{of acetic anhydride} so-lution prepared in accordance with the Polish Standard PN­88/C­89082. The solution was refluxed for 30 mi­ nutes. After that, 1 cm3 _{of pyridine was added and} heat-ing continued for 10 minutes. Thereafter, 50 cm3 _of dis-tilled water was added, the mixture was cooled to room temperature and titrated with the use of a standard so-lution of potassium hydroxide KOH in distilled water (0.5 mol/dm3_{) and phenolphthalein as indicator.}

Based on the results of the end-groups determination, the average molecular weights of bio­based polyols were calculated from following Equation (5):

(5)

where: M_n – average molecular weight calculated with the use of end-groups method [g/mol], f – bio-based poly-ols functionality, established value f = 2 [-], 56.1 – molar mass of the potassium hydroxide [g/mol], L_k – acid num-ber [mg KOH/g], L_OH – hydroxyl number [mg KOH/g].

Dynamic viscosity

Dynamic viscosity measurements were performed with the use of rotary rheometer R/S­CPS+ produced by Brookfield Company, USA. The viscosity values at 80 °C were defined with the use of computer program Rheo3000. Measurements were conducted with con-trolled shear rate (CSR). The justification of the choice of temperature was based on temperature ranges in some industrial processes.

1_{H NMR spectroscopy}

Proton nuclear magnetic resonance (1_{H NMR)} spectros-copy was carried out with the use of Bruker spectrome-ter. Operating frequency was 400 MHz for protons. The

ca. 10 % w/v solutions of the poly(propylene succinate)

polyesters were prepared in a CDCl₃ solvent at ambient temperature. The simulation and iteration of the obtained spectra were carried out using Bruker software.

Thermal characteristic

DSC­TG/QMS coupled method of thermogravimetric measurements were conducted using an STA 449 F1 Jupiter apparatus from NETZSCH­Feinmahltechnik GmbH Germany. Approximately 20 mg portions of dry samples were placed in corundum crucible and heated to 650 °C at various rates: 10, 15 and 20deg/min and under helium flow. Under these conditions, the DSC­TG experiments of bio-based polyester polyols were sufficiently reproducible.

RESULTS AND DISCUSSION

Synthesis and characterization of the poly(propylene succinate)s

All prepared polyester polyols (PPS) were synthesized with the use of well­known two­step polycondensation method. The first step was the esterification reaction, which was conducted for 10 hours for all of the prepared polyester polyols without the use of catalyst. After re-moval of minimum 60 % water, the catalyst was added. The second step, which was the main polycondensation reaction, was carried out for individual time periods for all synthesized polyesters until achievement the acid number ca. or preferably lower than 1 mg KOH/g. The justification of the choice of end-point of the polyconden-sation reaction was based on the content of carboxyl end-groups corresponding to the acid number determined for some synthetic polyester polyols commonly used in the

polyurethane industry. Table 1 shows properties of the prepared polyols.

The conducted synthesis confirmed the influence of temperature conditions during both steps of the poly-condensation on the reaction kinetics. Table 1 presents the shortest reaction time for polyols prepared at

high-est temperature conditions. The reaction time for PPS 140/200 was 13 hours. The synthesis was carried out un-til the acid number of the resulting polyol reached ca. 1 mg KOH/g. The hydroxyl number increased with the increasing synthesis temperature and the highest value was determined for polyol PPS 160/180. One of the most important properties for the industry, which gives infor-mation about probable behavior of polyol during indus-trial processing is viscosity. The lowest value at 2.76 Pa · s was measured for PPS 140/200.

1_{H NMR spectroscopy}

The structure analysis of received products was per-formed using 1_{H NMR measurements. The resulted} spectra verified that poly(propylene succinate)s were obtained. Figure 1 shows the exemplary 1_{H NMR} spec-trum of PPS 160/180. The characteristic intensive single peak at 2.63 ppm is attributed to methylene protons from succi nic acid [­CH₂­C(O)­] [38]. Peaks at 4.20 and 2.00 ppm are connected with a triple and multiple peaks corresponding to methylene protons from propylene glycol (1,3­propanediol), (­CH₂­O­) and (­CH₂-),

respec-T a b l e 1. Properties of the prepared polyester polyols

Sample

Synthesis temperature

°C Reaction time

h mg KOH/gLk mg KOH/gLOH g/molMn

Viscosity at 80°C Pa · s I step II step PPS 140/160 140 160 18 0.83 51.5 2200 3.43 PPS 140/190 190 16 1.05 58.5 1900 4.66 PPS 140/200 200 13 1.02 77.4 1400 2.76 PPS 150/180 150 180 17 0.96 63.4 1800 7.41 PPS 150/190 190 16 1.15 48.7 2300 3.47 PPS 150/200 200 14 0.80 70.4 1600 3.38 PPS 160/180 160 180 16 1.05 79.0 1400 4.76 PPS 160/190 190 14 1.00 64.7 1700 5.77 PPS 160/200 200 14 1.02 71.8 1600 4.34 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 , ppm -CH -O-2 -CH -2 -CH -C(O)-2 PPS 160/180 Fig. 1. 1_{H NMR spectrum of PPS 160/180}

T a b l e 2. Thermal degradation characteristics of the prepared polyester polyols at heating rate of 10 deg/min

Sample

Thermal degradation characteristics

T 5 % °C T 50 % °C T 90 % °C T max °C Residue at 650 °C % PPS 140/160 323.2 393.9 415.5 403.2 0.35 PPS 140/190 323.8 394.1 417.7 403.9 0.67 PPS 140/200 326.0 395.9 418.5 401.0 0.96 PPS 150/180 320.8 395.9 418.5 401.0 0.99 PPS 150/190 327.5 395.6 418.4 403.6 0.67 PPS 150/200 318.2 396.0 417.2 403.2 0.94 PPS 160/180 318.9 394.9 417.8 403.9 0.61 PPS 160/190 321.0 395.0 417.3 401.3 0.30 PPS 160/200 313.2 395.2 417.1 403.2 0.89

tively [35]. More precise description can be found in ref-erences [39, 40].

Thermogravimetric analysis

Thermogravimetric analyses were performed to evalu-ate the thermal properties of the synthesized mevalu-aterials. The results confirmed their high thermal stability. Table 2 presents the characteristic temperatures of thermal de-composition of the prepared materials measured at the heating rate 10 deg/min. The characteristic temperatures of 5, 50 and 90 % of the weight loss and the temperature of the maximum rate of weight loss for all samples revealed similar values. The lowest value of T_{5 %} at 313.2 °C which gives information about the beginning of the thermal de-composition was determined for PPS 160/200. The highest value of the T_{5 %} at 327.5 °C was observed for PPS 150/190. The temperature of the maximum rate of weight loss was within the range from 401.0 to 403.9 °C.

Figures 2 and 3 present the TGA and DTG graphs for relevant materials. The results confirmed one­step mechanism of the thermal degradation and similarity of the thermal stability characteristics. The most visible differences are related to the intensity of the DTG curves what gives information about the rate of mass loss of materials.

Thermal degradation kinetics

Thermogravimetric analyses were conducted at dif-ferent heating rates to determine the kinetics of ther-mal degradation. Table 3 presents the values of activa-tion energy of polyester polyol decomposiactiva-tion calculated with the use of two primary methods: Kissinger as well as Ozawa, Flynn and Wall (OFW). The presented find-ings demonstrate that activation energy determined by OFW method shows distinct dependence on the synthe-sis temperature conditions. An increase in activation en-ergy values calculated with this method was observed with increasing temperature conditions during the

syn-thesis of polyols. Only for polyols prepared at 160 °C of the first step of the polycondensation, the activation en-ergy decreased with the increase of the second step tem-perature. The highest E_a value was 196.4 kJ/mol for PPS 160/180. The lowest E_a equal to 154.2 kJ/mol was deter-mined for the polyol PPS 150/180. In the case of Kissinger method results, there is no clear dependence of synthesis temperature conditions and thermal degradation kinet-ics. For the comparison between both methods and more precise study, the partitive activation energies for three selected polyols were calculated with the use of OFW method. Figures 4, 5 and 6 present the Ozawa, Flynn and Wall plots of the selected synthesized polyesters. The straight lines are given which slope is proportional to the activation energy (­E_a/R). When the activation energy

E_a increases with the increase of the conversion degree, the complex reaction mechanism can be confirmed. The single­step reaction can be verified if the determined ac-tivation energy E_a is the same for the different α conver-sion values [35, 41, 42]. Figure 7 shows the dependence between the activation energy E_a and degradation con-version α for PPS 140/160, PPS 150/180 and PPS 160/180. It

100 90 80 70 60 50 40 30 20 10 0 100 200 300 400 500 600 Temperature, °C m , % PPS 140/160 PPS 140/190 PPS 140/200 PPS 150/180 PPS 150/190 PPS 150/200 PPS 160/180 PPS 160/190 PPS 160/200 0 -5 -10 -15 -20 d /d ,%/mi n m t 100 200 300 400 500 600 Temperature, °C PPS 140/190 PPS 140/200 PPS 150/180 PPS 150/190 PPS 150/200 PPS 160/180 PPS 140/160 PPS 160/190 PPS 160/200 _{380 390 400 410} -20 -19 -18 -17 -16 -15

Fig. 2. TGA graph of the synthesized polyester polyols Fig. 3. DTG graph of the synthesized polyester polyols

T a b l e 3. Activation energy (Ea) of thermal decomposition of the synthesized polyester polyols determined by Ozawa, Flynn and Wall as well as Kissinger methods

Sample

E_a, kJ/mol Ozawa, Flynn and

Wall method Kissinger method

PPS 140/160 164.3 185.7 PPS 140/190 169.1 211.0 PPS 140/200 180.6 144.2 PPS 150/180 154.2 135.5 PPS 150/190 165.8 149.7 PPS 150/200 173.6 188.1 PPS 160/180 196.4 174.7 PPS 160/190 190.8 182.8 PPS 160/200 181.2 149.0

is shown that with the increase of the conversion degree, the activation energy also increases.

After the conversion reached 0.8 < α < 0.9, the E_a de-creased (see Fig. 7). Therefore, the results verified the existence of the multi-stage reaction during the thermal decomposition of the prepared polyols.

The conducted investigations allowed to verify the lower value of activation energy for fully bio­based poly(propylene succinate) than those of their petro-chemical­based counterparts. Chrissafis et al. [35] and Bikiaris et al. [43] determined the activation energy of the petrochemical-based poly(propylene succinate) at ca. 220 kJ/mol.

CONCLUSIONS

A series of the linear bio­based polyester polyols were synthesized with the use of two-step polycondensa-tion reacpolycondensa-tion conducted under the different temperature conditions. The reaction conditions and reagents ratio were selected for polyol production in accordance with the requirements of thermoplastic polyurethane indus-try. The differences of macromolecular structure were determined based on the values of acid and hydroxyl numbers, average molecular weights and viscosities. The thermal degradation kinetics of the synthesized polyols was also investigated. The results indicate the differences in the activation energy calculated with the use of two primary methods: Ozawa, Flynn and Wall as well as Kissinger. Based on the OFW method a dis-tinct dependence between bio-based polyols synthesis conditions and kinetics of their thermal degradation was found. With the increasing temperature conditions, the increasing activation energy was observed. Only for polyols prepared at 160 °C at the first step of polycon-densation revealed decreasing E_a with elevated tempe­ rature during the second step of polycondensation. The highest thermal degradation activation energy by OFW method and the most similar to that of petrochemical-­based poly(propylene succinate), equal to 196.4 kJ/mol, was determined for polyol PPS 160/180. Moreover, the results verified the existence of the multi­stage reaction during thermal decomposition of the prepared polyols. The comparison between primary properties and activa-tion energies allowed to select PPS 140/200 as one of the most suitable for use as a polyol for polyurethanes. The critical information was the lowest viscosity value and one of the highest activation energies from all of the pre-pared bio-based polyols.

ACKNOWLEDGMENTS

The authors gratefully acknowledge receiving the samples of succinic acid used in this study from BioAmber Sarnia Inc. (Canadian corporation). The sincere acknowledgments are also directed for the DuPont Tate&Lyle Corporation for supplying the glycol (1,3-propanediol) samples used in this study.

1/T · 10 , 1/K-3 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95 1.35 1.30 1.25 1.20 1.15 log  1.10 1.05 1.00 0.95 1.38 1.46 1.54 1.62 1.70 1.78 1.86 PPS 140/160 0.05 0.20 0.95 0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95 1.35 1.30 1.25 1.20 1.15 log  1.10 1.05 1.00 0.95 1.38 1.46 1.54 1.62 1.70 1.78 1.86 PPS 150/180 0.05 0.20 0.95 1/ · 10 , 1/KT -3 1.35 1.30 1.25 1.20 1.15 log  1.10 1.05 1.00 0.95 1.38 1.46 1.54 1.62 1.70 1.78 1.86 1/ · 10 , 1/KT -3 PPS 160/180 _0.05 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 0.95 0.05 0.20 0.95 200 180 160 140 120 100 80 60 40 20 0 Ea ,kJ/mol 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0  PPS 140/160 PPS 150/180 PPS 160/180 Fig. 4. Ozawa, Flynn and Wall plots of PPS 140/160

Fig. 5. Ozawa, Flynn and Wall plots of PPS 150/180

Fig. 6. Ozawa, Flynn and Wall plots of PPS 160/180

Fig. 7. Dependence of activation energy E_a and degradation con-version α for PPS 140/160, PPS 150/180 and PPS 160/180, deter-mined by OFW method

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